As a toner for printing, a color-changing toner, which usually exhibits a specific color and is changed to become colorless or transparent after being irradiated by near-infrared rays, is known, and there are suggested several kinds of image processing apparatuses using the toner for printing. For example, Unexamined Japanese Patent Application Publication No. 1993-69675 (hereinafter referred to as Patent Document 1) discloses a copying machine utilizing a color-changing toner to copy an original document. Also, Unexamined Japanese Patent Application Publication No. 2008-9312 (hereinafter referred to as Patent Document 2) discloses an image forming apparatus and the like utilizing normal toner and color-changing toner to form an image, wherein a prescribed identification mark is printed on a sheet on which an image is formed by color-changing toner, and discriminates the sheet on which an image is formed by color-changing toner by detecting existence or nonexistence of the identification mark, in order to recycle the sheet printed with that color-changing toner.
The above mentioned color-changing toner may be used to print a confidential document in order to prevent confidential information from leaking by making it possible to erase the content of documents. However, even though a copying machine disclosed in Patent Document 1 is used to make a copy of a confidential document by utilizing color-changing toner, for example, if the document, which is duplicated by color-changing toner, is copied by a copying machine which uses a normal toner, then a non-erasable document is reproduced from the document and it becomes possible to compromise the information.
Also, if a technique disclosed by Patent Document 2, in which a sheet on which an image has been formed by a color-changing toner is discriminated by detecting existence or nonexistence of an identification mark, is used to prevent a confidential document, which has been printed with color-changing toner, from being duplicated, in other words, if a technique by which document duplication by normal toner is prohibited in case an identification mark is detected is used, it is still difficult to prevent information leakage because duplication with normal toner is possible by covering up or erasing the identification mark.